Problem: Solve for $x$ and $y$ using substitution. ${4x-y = -9}$ ${y = -x-11}$
Since $y$ has already been solved for, substitute $-x-11$ for $y$ in the first equation. ${4x - }{(-x-11)}{= -9}$ Simplify and solve for $x$ $4x+x + 11 = -9$ $5x+11 = -9$ $5x+11{-11} = -9{-11}$ $5x = -20$ $\dfrac{5x}{{5}} = \dfrac{-20}{{5}}$ ${x = -4}$ Now that you know ${x = -4}$ , plug it back into $\thinspace {y = -x-11}\thinspace$ to find $y$ ${y = -}{(-4)}{ - 11}$ $y = 4 - 11$ $y = -7$ You can also plug ${x = -4}$ into $\thinspace {4x-y = -9}\thinspace$ and get the same answer for $y$ : ${4}{(-4)}{ - y = -9}$ ${y = -7}$